Re: [PULL 3/9] meson: Disallow 64-bit on 32-bit Xen emulation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/02/2025 11:20 am, Philippe Mathieu-Daudé wrote:
> Hi,
>
> Adding Xen community.
>
> On 8/2/25 21:57, Richard Henderson wrote:
>> Require a 64-bit host binary to spawn a 64-bit guest.
>>
>> Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
>> Signed-off-by: Richard Henderson <richard.henderson@xxxxxxxxxx>
>> ---
>>   meson.build | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/meson.build b/meson.build
>> index 1af8aeb194..911955cfa8 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -304,9 +304,14 @@ else
>>   endif
>>   accelerator_targets = { 'CONFIG_KVM': kvm_targets }
>>   -if cpu in ['x86', 'x86_64']
>> +if cpu == 'x86'
>> +  xen_targets = ['i386-softmmu']
>> +elif cpu == 'x86_64'
>>     xen_targets = ['i386-softmmu', 'x86_64-softmmu']
>> -elif cpu in ['arm', 'aarch64']
>> +elif cpu == 'arm'
>> +  # i386 emulator provides xenpv machine type for multiple
>> architectures
>> +  xen_targets = ['i386-softmmu']
>
> Is actually someone *testing* this config? I'm having hard time building
> it, so am very suspicious about how it runs, and start to wonder if I'm
> not just wasting my time (as could be our CI).

It was intentional.  I believe it was Stefano (CC'd) who introduced it.

Xen uses qemu-system-i386 everywhere because qemu-system-x86_64 doesn't
make compatible VMs.  I'm not sure why; I suspect it's bugs in the Xen
machine types, but I don't know QEMU well enough to be sure.

Another thing that (at least, was) tied to qemu-system-i386 was using
Qemu as a XenBlk <-> QCOW adapter, at which point it wasn't even really
a system emulator, just a paravirtual disk implementation.

This is, AIUI, what ARM wants with the xenpv machine.  If there's a
better way to do this, please do say.


Looking through Xen's CI, I can't see any of the ARM builds building
QEMU at all.  I think it's quite possible it's not tested any more.

~Andrew